The present invention relates to an interference compensation system which cancels the undesired interference signal in a wireless communication receiver. The present invention is utilized in the earth station for a satellite communication system to which a conventional communication system gives an undesired interference signal, and of course, the present invention is utilized for a wireless communication system in general, the application of which is not restricted to a satellite communication system.
A prior system for cancelling an interference signal is shown in FIG. 1. The typical prior system is shown in British magazine "Systems Technology" No. 32, 1979 September pages 38-47, by T. A. Bristow. In FIG. 1, the reference numeral 1 is a main antenna which receives both the desired signal and the undesired interference signal, 2 is an auxiliary antenna which receives only an undesired interference signal, 3 is an amplitude-phase control circuit, 4 is a combiner, 5 is a correlation detector, 100 and 101 are frequency converters each of which includs an amplifier, 102 is a low-pass filter, and 15 is an output terminal which provides the output signal which is free from an interference signal. In FIG. 1, the main antenna 1 is directed to a desired signal, but receives both the desired signal and the undesired interference signal. The auxiliary antenna 2 is directed to the undesired interference signal, and receives only the interference signal. The interference signal received by the auxiliary antenna 2 is applied to the amplitude-phase control circuit 3 which adjusts the amplitude and the phase of the interference signal so that the output of the control circuit 3 has the equal amplitude and the anti-phase as that of the interference signal received by the main antenna 1. Thus, when the combiner 4 combines the interference signal from the main antenna with the output of the amplitude-phase control circuit 3, the interference signal is cancelled, and the desired signal is obtained at the output of the combiner. The control signal for controlling the amplitude-phase control circuit 3 is obtained by the correlation detector 5, which provides the correlation between the undesired signal received by the main antenna 1 and the undesired interference signal received by the auxiliary antenna 2. In a practical embodiment, frequency converters 100 and 101 are provided between the correlation detector 5 and each of the antennas, and a low-pass filter 102 is provided between the correlation detector 5 and the control circuit 3. Usually, the frequency converters include an amplifier. The correlation detector 5 is usually implemented by a phase detector, or a frequency mixer. It should be noted that the prior system of FIG. 1 obtains directly the correlation between the residual interference component at the output of the combiner 4 and the interference signal received by the auxiliary antenna 2 from the amplitude and the phase of the carrier signal. Accordingly, when a PSK signal (Phase Shift Keying signal) in which the phase of the signal changes rapidly according to the modulation signal is concerned, the difference between paths of the signals to the two antennas must be smaller than the predetermined value, because the output of the phase detector changes uncontinuously when the path difference is large.
However, due to the phase fluctuation of the previous stages of the correlation detectors 5 and the path fluctuation in a transmission path to each antenna when the sudden large change of the phase difference between main and auxiliary channels occurs, the interference compensation is almost impossible when a PSK signal is concerned. Further, when an FM signal is concerned, the compensation effect is not large, because no correlation between phase deviations of each signal exists when the path difference is large, and the accurate correlation is not obtained.
Further, it should be appreciated in FIG. 1 that a pair of frequency converters 100 and 101 must be provided in a prior art, and the characteristics including the amplitude characteristics and the phase characteristics of those frequency converters must be exactly the same as each other. However, the exact coincidence of two frequency converters in each ambient temperature is almost impossible.